Cable television systems utilize a central provider of program information which serves a large number of end users generally referred to as subscribers. The central provider portion of the cable television system usually called the "headend" provides a plurality of program information as well as other information to the subscribers via a multiple branch distribution network which may define several tiers of distribution facilities.
In addition to actual programming information, the cable television system is required to carry additional management and operating data provided by the headend to the large number of end users or subscribers. Communication from the headend to the subscribers is generally referred to as "downstream" communication. In some cable television systems referred to as one-way, all information and data is transferred downstream. In other cable television systems referred to as two-way systems, communication is also provided from the various subscribers throughout the network to the headend in what is referred to as "upstream" communication. Examples of subscriber originated upstream information may include program purchasing requests, opinion poll responses, and subscriber terminal status information.
Because of the large number of subscribers being serviced by the headend, the capacity of the system to provide upstream communication is correspondingly limited. As a large number of subscriber terminals attempt to communicate upstream with the same headend facility, the system's capacity to simultaneously receive such communications is quickly exceeded. As a result, in all two-way cable television systems, some form of allotment or conflict resolution is applied to upstream communication. Upstream communication in cable television systems is provided using out-of-band communication. Such out-of-band communication uses a separate communication capability outside the standard television broadcast channel and thus is not limited to the vertical blanking interval or other time restraint. Instead, a separate carrier, usually between five and thirty megahertz, is modulated with upstream data at the decoder and transmitted to the headend.
In virtually all two-way cable television systems, certain types of upstream communication are extremely important if not essential and therefore cannot be unduly restricted or delayed by conflict resolution systems without degrading the effectiveness of the entire cable television system. Thus, such effective operation of cable television systems requires that a certain amount of guaranteed access for upstream communication must be provided to each subscriber terminal.
U.S. Pat. No. 4,553,161 issued to Citta sets forth an UPSTREAM DATA PACKET TIME SLOT SYNCHRONIZATION WITH DOWNSTREAM VBI FOR TWO-WAY CATV SYSTEM in which synchronization of upstream data packet time slots with the vertical blanking interval of a video signal transmitted downstream from the headend is provided in a two-way cable television system. Coded data transmitted during the designated vertical blanking interval from the headend is compared with system data stored in each subscriber terminal for generating a match signal. The match signal is used to interrupt a microcomputer which then divides the next video signal raster scan field into equal time slots during any one of which an upstream data packet may be transmitted.
While the prior art systems such as the above-described system in U.S. Pat. No. 4,553,161 have provided a form of guaranteed minimal upstream access for two-way cable television systems, there remains nonetheless a continuing need in the art for evermore improved upstream data transmission systems which provide flexibility and dynamic response capability as the operating circumstances of the cable television system change.
Accordingly, it is a general object of the present invention to provide an improved upstream data transmission system for cable television. It is a more particular object of the present invention to provide an improved upstream data transmission system for cable television which is able to respond dynamically to operating changes within the cable television system using headend controlled programmable system parameters.